This invention pertains generally to the preparation of electrodes for very high capacitance electrolytic double layer capacitors (super capacitors) and more particularly, to a method for preparation of high surface area carbon film electrodes for super capacitors.
Double layer capacitors are well known in the art and consist of an electrochemical cell, or combination of cells, including two electrodes, an electrolyte and a containment means. The electrolyte must provide good ionic conductivity but negligible electronic conductivity and can be an aqueous solution; a nonaqueous inorganic solution, that can comprise such liquids as thionyl chloride or sulfuryl chloride as solvents; or an organic solution that can comprise, for example, propylene carbonate, ethylene carbonate, acetonitrile, tetrahydrofuran, dimethylsulfoxide as solvents. The electrodes are polarizable and therefore, within a certain range of potentials, do not allow the exchange of electrons between the solute ions in the electrolyte and the electrode and generally consist of carbon or one or more oxides of ruthenium, tantalum, rhodium, iridium, cobalt, nickel, molybdenum, tungsten, or vanadium deposited onto a metal backing. As potential is applied between the electrodes of a double layer capacitor, ions from the electrolyte are adsorbed onto the electrodes causing an electrical double layer to be developed between the electrodes and the electrolyte into which they are immersed. This electrical double layer can be regarded as equivalent to a parallel plate capacitor wherein the capacitance is directly related to the surface area of the electrodes.
Because the capacitance of super capacitors is significantly greater, per unit volume than the more conventional parallel plate capacitor, super capacitors have found application in home audio/video equipment and computers as power back-up. Moreover, with the potential for capacitances as high as 900 F for carbon electrodes, super capacitors are extremely attractive as energy storage devices for electrical energy, particularly for load leveling applications such as electrical vehicles.
Since the capacitance of super capacitors is directly related to the surface area of the electrode, the need for super capacitors with higher volumetric capacitances (greater than 200 F./cc) has provided the impetus for developing electrode materials with very high surface areas. Various materials have been studied for super capacitor electrodes and the best of these materials have volumetric capacitances between 100 and 300 F./cc. However, most of these materials use either high cost polymer films and/or are produced through complicated laboratory fabrication and activation processes that are difficult to put into industrial mass production. What is needed is a low cost, high performance material (volumetric capacitance exceeding 200 F./cc) material for electrodes for super capacitor applications that can be fabricated inexpensively and on an industrial scale.